The zenith of the disease smallpox and its eradication in 1977 from human populations occurred prior to the modern era of immunology and molecular biology. Consequently there is little knowledge concerning the immune correlates for recovery from smallpox or the cross-reactive proteins expressed by baccinia virus that were responsible for its success as the smallpox vaccine. The only vaccination indicator that correlated with protection from severe smallpox was the scar. In response to the threat of bioterrorism, the U.S. government has redoubled its efforts to provide strategies that will protect the American public from an outbreak of smallpox or human monkeypox. As part of a comprehensive, multi-faceted plan, the U.S. government has contracted with Acambis Inc. and Baxter Healthcare Corp. to produce approximately 209 million doses of a new tissue culture smallpox vaccine. In addition, proposals are being considered for the next generation of smallpox vaccine that will have an enhanced safety profile, causing fewer vaccine-related complications, especially in immunosuppressed individuals. Evaluating the efficacy of the new Acambis and Baxter vaccine or other second-generation vaccines with enhanced safety profiles will be problematic without detailed knowledge of the immunogenicity of vaccines proven to be efficacious in the smallpox eradication program. Detailed studies on human B and T cell immune responses to proteins encoded by vaccinia virus should help fill this gap in knowledge, and may also identify targets of neutralizing, complement-fixation or ADCC (antibody-dependent cell cytotoxity) antibodies, which may facilitate the development of an efficacious replacement for VIG. We propose to characterize the vaccinia virus-encoded proteins recognized by B and T cell responses during the vaccination of volunteers with the DryVax vaccine. The Specific Aims are to: 1. Characterize the antibody responses to immunodominant vaccinia virus proteins and 2. Identify the epitope specificity of representative vaccinia virus-specific T cell clones.